The present invention is related to optical devices for fiberoptic systems and networks and, in particular, to WDM multiplexer/demultiplexers, i.e., WDM multiplexers, demultiplexers and add/drop multiplexers.
In WDM (Wavelength Division Multiplexing) fiberoptic systems and networks, the wavelength of optical signals is used to define a communication channel through the system. While the term, WDM, also refers to a specific ITU (International Telecommunications Union) standard, WDM is used herein in the former, more general, sense unless the stricter definition is specifically identified. A WDM multiplexer combines a plurality, typically all, of the channels into a single optical path and a WDM demultiplexer splits or separates a plurality, typically all, of the constituent channels in an optical path. A WDM add/drop multiplexer is a component in WDM networks with the functions of WDM multiplexing and/or demultiplexing; it allows optical signals of one or more particular wavelengths to be inserted, i.e., added, into an optical fiber and/or optical signals of one or more wavelengths to be diverted, i.e., dropped, from the optical fiber. The added and dropped signals can be assigned to one or more users with the added signals being transmitted by the user(s) to the optical network and the dropped signals being received by the user(s) from the network.
One example of a WDM add/drop multiplexer is an assembly of cascaded WDM couplers such as shown in FIG. 22 of U.S. Pat. No. 5,642,448, entitled “INTEGRABLE FIBEROPTIC COUPLER AND RESULTING DEVICES AND SYSTEMS,” and which issued Jun. 24, 1997 to J. J. Pan et al. In the drawing, each of the couplers “drops” optical signals of one of the wavelengths λ1–λ7, while signals at the wavelength λ8 are separated as the remainder after the other wavelengths have been removed. With a reversal in direction of all the light signals, the assembly can perform an “add” function for the optical signals at each of the wavelengths λ1–λ8. Nevertheless, this assembly manifests the problems of current WDM add/drop multiplexers; manufacturing costs are higher than desirable and the packaged device is bulky.
The present invention addresses these problems of a WDM add/drop multiplexer in particular, and, more generally, provides for WDM multiplexer/demultiplexers, i.e., WDM multiplexer, demultiplexer or WDM add/drop multiplexer devices, which are less costly to manufacture and which is miniature in size. Optical performance is good, especially for emerging CWDM (Coarse Wavelength Division Multiplexing) networks which are being used in Metropolitan Area Networks (MANs). In CWDM networks, the wavelength channels are spaced further apart than the ITU-defined Dense WDM, or DWDM, channel spacings to avoid or reduce many of the problems of tight channel spacing.